1. Field of the Invention
This invention generally relates to an optical pick-up device for use in an optical disc system or the like, and, in particular, to a split type optical pick-up device for writing and reading information to and from an optical disc, which includes a stationary optical system and a movable optical system.
2. Description of the Prior Art
An optical pick-up device of an optical disc memory system is well known, and, in general, such a device includes an objective lens through which a laser beam is passed to form a small light spot on an optical information recording medium, such as an optical disc, thereby allowing to record and read or erase information to and from the recording medium. Such an optical disc system is particularly large in capacity and an optical disc is removable.
In such an optical disc memory system, since an information pit, i.e., a unit of information to be recorded on an optical disc, is extremely small in the order of 1 micron, so that, in order to record and reproduce information to and from an optical disc accurately, it is necessary to carry out focusing control, tracking control and seek control as well known in the art. And, the focusing control is typically carried out by displacing the objective lens along an optical axis thereof, and the tracking control is carried out by displacing the objective lens in the tracking direction or in a direction transverse to a recording track of the optical disc. On the other hand, the seek control is typically carried out such that the overall optical pick-up device is once moved to a location in the vicinity of a target track by a coarse control operation and then a fine control operation is carried out.
However, as compared with the typical prior art magnetic disc memory device, an optical pick-up device weighs several hundreds grams in its entirety. For this reason, according to a control method in which the entire optical pick-up device is moved in the tracking direction, the inertia of the optical pick-up device comes into play during the seek control operation for bringing a laser spot into a position in the vicinity of a desired track by moving the objective lens. Because of this, a high-speed seek operation is difficult to carry out and thus the access time tends to be longer.
In order to cope with this problem, a split type or actuator seek type optical pick-up device is under development so as to realize high-speed accessing in an optical disc memory system. FIG. 5 schematically illustrates such a split type optical pick-up device. As shown in FIG. 5, such a split type optical pick-up device includes a movable optical system 2 which is located opposite to a recording surface of an optical disc 1 as an optical information recording medium and which executes a seek movement in the radial direction of the disc 1. Also provided as optically coupled to the movable optical system 2 is a stationary optical system 3 which is mounted on a mounting device or carriage as fixedly attached thereto. The movable optical system 2 includes a deflecting prism 4 for coupling a laser beam between the optical disc 1 and the stationary optical system 3 by deflecting the laser beam which has travelled in the horizontal direction (in parallel with the optical disc 1) into the vertical direction (toward the recording surface of the optical disc 1) and an objective lens 5 which is disposed opposite to the recording surface of the optical disc 1. On the other hand, the stationary optical system 3 includes a light source, typically a semiconductor laser, and a detector unit for detecting a focusing signal, a tracking signal or an RF signal for reading and reproducing stored data. It is to be noted that in FIG. 5 the tracking direction indicates the right and left direction or radial direction of the optical disc 1 with the optical disc 1 set in position in an optical disc memory system.
The split type optical system under development has a basic feature of separating the entire structure into the movable optical system 2 having the objective lens 5, movable with respect to the optical disc 1, and the stationary optical system 3, whereby the movable optical system 2 is provided to be movable with respect to the optical disc 1 in the tracking direction. With this structure, a laser beam emitted from the stationary optical system 3 is first incident upon the deflecting prism 4 of the movable optical system 2 and it is deflected toward the objective lens 5. And, the laser beam is focused onto the recording surface of the optical disc 1 in the form of a light spot having a predetermined size by means of the objective lens 5. Then, the reflecting light from the optical disc 1 travels the same optical path in the reversed direction to finally reach the stationary optical system 3. Thus, the information carried by this returning light is used to read information recorded on the optical disc 1 or to carry out focusing and tracking controls.
Now, referring to FIGS. 6a and 6b, the principle of operation for detecting a tracking signal will be described below. FIG. 6a illustrates the condition in which a laser light spot 6 condensed by the objective lens 5 is located at the center of a track 7 of the optical disc 1. In this case, since the light spot 6 is located at the center of the track 7, the laser beam is diffracted symmetrically with the track as a center of symmetry. As a result, a pair of light-receiving elements, photoelectric elements, 8a and 8b of a two-division type light-receiving unit 8 for detecting a tracking signal, which is disposed inside of the stationary optical system 3 receives diffracted light having a symmetrical light distribution pattern 9 as shown in FIG. 6a. As a result, outputs A and B from these light-receiving elements 8a and 8b,respectively, are equal in magnitude, i.e., A=B, which indicates the fact that the optical pick-up device is in a proper tracking position. On the other hand, if the location of the light spot 6 is shifted sideways from the centeral position of the track 7 as shown in FIG. 6b, the distribution pattern 9 of diffracted light becomes asymmetrical between right and left, so that there is produced a difference in magnitude between outputs A and B of the respective light-receiving elements 8a and 8b. Under the condition, a tracking control operation is carried out so as to make this difference between outputs A and B to zero.
In such a tracking control operation, in the case of a split type optical pick-up device, only the movable optical system 2, having a relatively small mass, is moved with respect to the optical disc 1, so that the energy for moving an object to effect tracking control may remain relatively low in level and it can be carried out at high speed, e.g., access time within 100 msec. However, such a split type optical pick-up device is not free from drawbacks. For example, in the case where the movable optical system 2 is movably mounted on a carriage, if the movable optical system 2 experiences a displacement in the vertical direction while the carriage moves or if the axis of movement of the carriage is not in parallel with the optical axis of the laser beam emitted from the stationary optical system 3, as the movable optical system 2 moves on the carriage, there is produced an offset in the resulting tracking signal. Described more in detail with this respect referring to FIGS. 7 and 8, while the deflecting prism 4 is located at its initial position indicated by the solid line in FIG. 7, there is obtained a distribution pattern 9 of diffracted light on the tracking detecting light-receiving unit 8 as indicated also by the solid line. Under the condition, a detection signal obtained from the light-receiving unit 8 is so set to have no offset by initial adjustment as shown in FIG. 8a. However, when the deflecting prism 4 has been displaced over a distance d upwardly in the vertical direction as indicated by the phantom line in FIG. 7 due to movement of the movable optical system 2 for focusing control, the light path for the laser beam reflecting from the optical disc 1 and advancing to the light-receiving unit 8 within the stationary optical system 3 through the objective lens 5 and deflecting prism 4 is also shifted in position upwardly over the corresponding distance d. As a result, the diffracted light distribution pattern 9 on the light-receiving unit 8 is also shifted in position as indicated by the phantom line. Accordingly, there is produced an offset in a detection signal obtained from the light-receiving unit 8 as compared with the initial condition as shown in FIG. 8b. With such a tracking detection signal having an offset, the laser light spot 6 will be located at a position away from the central position of the track 7 when a tracking control operation has been carried out, thereby producing an error having an amount delta as shown in FIG. 8b. Therefore, under such a condition, an accurate recording and reproducing operation cannot be carried out.